The invention relates to frequency converters, and particularly to stabilizing voltage of an intermediate circuit of frequency converters.
A frequency converter is a device which is typically used for controlling motors at a varying supply frequency, enabling the speed of the motor being controlled to be controlled. Such frequency converters comprise a rectifier bridge, a direct voltage intermediate circuit, and an inverter. The rectifier bridge rectifies the voltage of a supply network and supplies with a rectified voltage the direct voltage intermediate circuit. From this voltage of the direct voltage intermediate circuit, the inverter, in turn, forms a voltage which has controllable frequency and amplitude and which is to be supplied to a load.
A direct voltage intermediate circuit, i.e. a DC intermediate circuit, of a frequency converter comprises one or more capacitors provided for serving as energy storages and for evening out direct voltage of the intermediate circuit. The intermediate circuit also includes inductance, either as an actual intermediate circuit DC choke or as an AC choke, which may be reduced to the DC side.
When charging the intermediate circuit, the charging current is to be limited in one way or another. Otherwise, a large current charging the capacitance of the intermediate circuit causes an overcurrent release. The voltage of the intermediate circuit may be raised to a voltage value corresponding with the operational state e.g. by a half controlled rectifier bridge. In such a half controlled bridge, second branches of each input phase are provided with controllable semiconductors, most typically thyristors. By decreasing the flow angle of the thyristors, the voltage of the intermediate circuit may be made to increase in a controlled manner by using a charging current of a magnitude within allowable limits. During normal use these thyristors are constantly controlled to be open such that the operation of the rectifier resembles that of a diode bridge.
A transfer function of an LC circuit formed by the inductance and capacitance of the intermediate circuit is a quadratic equation having gain dependent on losses at the resonance frequency of the LC circuit. An inverter of a frequency converter may be thought to present a constant power load to the voltage intermediate circuit, i.e. when a voltage Uc of the voltage intermediate circuit increases, current taken from the intermediate circuit decreases. Such a circuit involves a danger of intermediate circuit voltage oscillation.
Intermediate circuit voltage oscillation at supply frequency causes on the supply side a direct current component, which saturates a supply transformer, which, in turn, further increases the intermediate circuit oscillation.
The voltage oscillation occurs particularly in connection with high-power frequency converters at a large load and only in certain conditions. The frequency of such problematic voltage oscillation is of the magnitude of the frequency of the supply network or close to it. Such voltage oscillation stresses the DC capacitors of the intermediate circuit, for example.